Slow Cooker Lamb Barbacoa

Name: Slow Cooker Lamb Barbacoa
Ingredients: 1 Spanish onion (halved); 3 tomatillos (husked, washed, halved); 2 tomatoes (halved); 2 garlic cloves (peeled); 4 guajillo chiles (stemmed); 4 ancho chiles (or cascabel, stemmed); 2 cups water; 2 tablespoons agave nectar; 1 tablespoon apple cider vinegar; 1/4 teaspoon ground cinnamon; 1/2 teaspoon dried Mexican oregano; 1 teaspoon kosher salt; 2 to 3 pounds boneless lamb shoulder; 2 tablespoons granulated tomato bouillon with chicken flavor ((recommend Knorr)); 6 ounces Mexican beer; 2 tablespoons vegetable oil